Introduction to How Nuclear Fusion Reactors Work

Fusion reactors have been getting a lot of press recently because they offer some major advantages over other power sources. They will use abundant sources of fuel, they will not leak radiation above normal background levels and they will produce less radioactive waste than current fission reactors.

Nobody has put the technology into practice yet, but working reactors aren't actually that far off. Fusion reactors are now in experimental stages at several laboratories in the United States and around the world.

A consortium from the United States, Russia, Europe and Japan has proposed to build a fusion reactor called the International Thermonuclear Experimental Reactor (ITER) in Cadarache, France, to demonstrate the feasibility of using sustained fusion reactions for making electricity. In this article, we'll learn about nuclear fusion and see how the ITER reactor will work.

Isotopes are atoms of the same element that have the same number of protons and electrons but a different number of neutrons. Some common isotopes in fusion are:

Protium is a hydrogen isotope with one proton and no neutrons. It is the most common form of hydrogen and the most common element in the universe.

Deuterium is a hydrogen isotope with one proton and one neutron. It is not radioactive and can be extracted from seawater.

Tritium is a hydrogen isotope with one proton and two neutrons. It is radioactive, with a half-life of about 10 years. Tritium does not occur naturally but can be made by bombarding lithium with neutrons.

Helium-3 is a helium isotope with two protons and one neutron.

Helium-4 is the most common, naturally occurring form of helium, with two protons and two neutrons.

Physics of Nuclear Fusion: Reactions

Current nuclear reactors use nuclear fission to generate power. In nuclear fission, you get energy from splitting one atom into two atoms. In a conventional nuclear reactor, high-energy neutrons split heavy atoms of uranium, yielding large amounts of energy, radiation and radioactive wastes that last for long periods of time (see How Nuclear Power Works).

In nuclear fusion, you get energy when two atoms join together to form one. In a fusion reactor, hydrogen atoms come together to form helium atoms, neutrons and vast amounts of energy. It's the same type of reaction that powers hydrogen bombs and the sun. This would be a cleaner, safer, more efficient and more abundant source of power than nuclear fission.

There are several types of fusion reactions. Most involve the isotopes of hydrogen called deuterium and tritium:

Proton-proton chain - This sequence is the predominant fusion reaction scheme used by stars such as the sun. Two pairs of protons form to make two deuterium atoms. Each deuterium atom combines with a proton to form a helium-3 atom. Two helium-3 atoms combine to form beryllium-6, which is unstable. Beryllium-6 decays into two helium-4 atoms. These reactions produce high energy particles (protons, electrons, neutrinos, positrons) and radiation (light, gamma rays)

Deuterium-deuterium reactions - Two deuterium atoms combine to form a helium-3 atom and a neutron.

Deuterium-tritium reactions - One atom of deuterium and one atom of tritium combine to form a helium-4 atom and a neutron. Most of the energy released is in the form of the high-energy neutron.

Conceptually, harnessing nuclear fusion in a reactor is a no-brainer. But it has been extremely difficult for scientists to come up with a controllable, non-destructive way of doing it. To understand why, we need to look at the necessary conditions for nuclear fusion.

Conditions for Nuclear Fusion

­W­hen hydrogen atoms fuse, the nuclei must come together. However, the protons in each nucleus will tend to repel each other because they have the same charge (positive). If you've ever tried to place two magnets together and felt them push apart from each other, you've experienced this principle first-hand.

To achieve fusion­, you need to create special conditions to overcome this tendency. Here are the conditions that make fusion possible:

High temperature - The high temperature gives the hydrogen atoms enough energy to overcome the electrical repulsion between the protons.

Fusion requires temperatures about 100 million Kelvin (approximately six times hotter than the sun's core).

At these temperatures, hydrogen is a plasma, not a gas. Plasma is a high-energy state of matter in which all the electrons are stripped from atoms and move freely about.

The sun achieves these temperatures by its large mass and the force of gravity compressing this mass in the core. We must use energy from microwaves, lasers and ion particles to achieve these temperatures.

High pressure - Pressure squeezes the hydrogen atoms together. They must be within 1x10-15 meters of each other to fuse.

The sun uses its mass and the force of gravity to squeeze hydrogen atoms together in its core.

We must squeeze hydrogen atoms together by using intense magnetic fields, powerful lasers or ion beams.

­W­ith current technology, we can only achieve the temperatures and pressures necessary to make deuterium-tritium fusion possible. Deuterium-deuterium fusion requires higher temperatures that may be possible in the future. Ultimately, deuterium-deuterium fusion will be better because it is easier to extract deuterium from seawater than to make tritium from lithium. Also, deuterium is not radioactive, and deuterium-deuterium reactions will yield more energy.

Plasma toroid

Fusion Reactors: Magnetic Confinement

There are two ways to achieve the temperatures and pressures necessary for hydrogen fusion to take place:

Magnetic confinement uses magnetic and electric fields to heat and squeeze the hydrogen plasma. The ITER project in France is using this method.

Inertial confinement uses laser beams or ion beams to squeeze and heat the hydrogen plasma. Scientists are studying this experimental approach at the National Ignition Facility of Lawrence Livermore Laboratory in the United States.

Let's look at magnetic confinement first. Here's how it would work:

Microwaves, electricity and neutral particle beams from accelerators heat a stream of hydrogen gas. This heating turns the gas into plasma. This plasma gets squeezed by super-conducting magnets, thereby allowing fusion to occur. The most efficient shape for the magnetically confined plasma is a donut shape (toroid).

A reactor of this shape is called a tokamak. The ITER tokamak will be a self-contained reactor whose parts are in various cassettes. These cassettes can be easily inserted and removed without having to tear down the entire reactor for maintenance. The tokamak will have a plasma toroid with a 2-meter inner radius and a 6.2-meter outer radius.

Let's take a closer look at the ITER fusion reactor to see how magnetic confinement works.

Tokamak

"Tokamak" is a Russian acronym for "toroidal chamber with axial magnetic field."

ITER tokamak

Courtesy ITER

Magnetic Confinement: The ITER Example

The main parts of the ITER tokamak reactor are:

Vacuum vessel - holds the plasma and keeps the reaction chamber in a vacuum

Neutral beam injector (ion cyclotron system) - injects particle beams from the accelerator into the plasma to help heat the plasma to critical temperature

Transformers/Central solenoid - supply electricity to the magnetic field coils

Cooling equipment (crostat, cryopump) - cool the magnets

Blanket modules - made of lithium; absorb heat and high-energy neutrons from the fusion reaction

Divertors - exhaust the helium products of the fusion reaction

Here's how the process will work:

Magnetic-confinement fusion process

The fusion reactor will heat a stream of deuterium and tritium fuel to form high-temperature plasma. It will squeeze the plasma so that fusion can take place. The power needed to start the fusion reaction will be about 70 megawatts, but the power yield from the reaction will be about 500 megawatts. The fusion reaction will last from 300 to 500 seconds. (Eventually, there will be a sustained fusion reaction.)

The lithium blankets outside the plasma reaction chamber will absorb high-energy neutrons from the fusion reaction to make more tritium fuel. The blankets will also get heated by the neutrons.

The heat will be transferred by a water-cooling loop to a heat exchanger to make steam.

Fusion Reactors: Inertial Confinement

­The National Ignition Facility (NIF) at Lawrence Livermore Laboratory is exp­erimenting with using laser beams to induce fusion. In the NIF device, 192 laser beams will focus on single point in a 10-meter-diameter target chamber called a hohlraum. A hohlraum is "a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity" (Science & Engineering Encyclopaedia).

At the focal point inside the target chamber, there will be a pea-sized pellet of deuterium-tritium encased in a small, plastic cylinder. The power from the lasers (1.8 million joules) will heat the cylinder and generate X-rays. The heat and radiation will convert the pellet into plasma and compress it until fusion occurs. The fusion reaction will be short-lived, about one-millionth of a second, but will yield 50 to 100 times more energy than is needed to initiate the fusion reaction. A reactor of this type would have multiple targets that would be ignited in succession to generate sustained heat production. Scientists estimate that each target can be made for as little as $0.25, making the fusion power plant cost efficient.

Like the magnetic-confinement fusion reactor, the heat from inertial-confinement fusion will be passed to a heat exchanger to make steam for producing electricity.

Cold Fusion

In 1989, researchers in the United States and Great Britain claimed to have made a fusion reactor at room temperature without confining high-temperature plasmas. They made an electrode of palladium, placed it in a thermos of heavy water (deuterium oxide) and passed an electrical current through the water. They claimed that the palladium catalyzed fusion by allowing deuterium atoms to get close enough for fusion to occur. However, several scientists in many countries failed to get the same result.

But in April 2005, cold fusion got a major boost. Scientists at UCLA initiated fusion using a pyroelectric crystal. They put the crystal into a small container filled with hydrogen, warmed the crystal to produce an electric field and inserted a metal wire into the container to focus the charge. The focused electric field powerfully repelled the positively charged hydrogen nuclei, and in the rush away from the wire, the nuclei smashed into eachother with enough force to fuse. The reaction took place at room temperature. See Coming in out of the cold: Cold fusion, for real (csmonitor.com) to learn more.

Applications of Fusion

­The main application for fusion is in making electricity. Nuclear fusion can pro­vide a safe, clean energy source for future generations with several advantages over current fission reactors:

Abundant fuel supply - Deuterium can be readily extracted from seawater, and excess tritium can be made in the fusion reactor itself from lithium, which is readily available in the Earth's crust. Uranium for fission is rare, and it must be mined and then enriched for use in reactors.

Safe - The amounts of fuel used for fusion are small compared to fission reactors. This is so that uncontrolled releases of energy do not occur. Most fusion reactors make less radiation than the natural background radiation we live with in our daily lives.

Clean - No combustion occurs in nuclear power (fission or fusion), so there is no air pollution.

Less nuclear waste - Fusion reactors will not produce high-level nuclear wastes like their fission counterparts, so disposal will be less of a problem. In addition, the wastes will not be of weapons-grade nuclear materials as is the case in fission reactors.

­NASA is currently looking into developing small-scale fusion reactors for powering­ deep-space rockets. Fusion propulsion would boast an unlimited fuel supply (hydrogen), would be more efficient and would ultimately lead to faster rockets.

For more information on nuclear fusion reactors and related topics, check out the links on the next page.